1. Field of the Invention
The present invention relates to active carbon and a process for its production. The active carbon produced by the present invention has micropores present in carbon particles and mesopores formed by linkage of carbon particles, and thus is useful in a technical field of e.g. electrodes for electrochemical capacitors, electrodes for gas sensors, negative electrode materials for lithium secondary cells, water treatment, sewage treatment, waste water treatment, waste gas treatment or carriers for catalysts, where the rate of adsorption is very much influenced by diffusion at the interior of the adsorbent. Further, it is excellent in gas permeability and thus useful as a separator for a gas filter or the like.
2. Discussion of Background
Usually, carbon having a large specific surface area active carbon is meant for in many cases. Active carbon is porous carbon having mainly micropores, and the porosity is provided by activating a solid.
The rate of adsorption is an important index for evaluating the adsorbing ability of an adsorbent such as active carbon. The rate of adsorption is substantially governed by the rate of diffusion at the interior of the adsorbent. Conventional active carbon did not have mesopores very much, whereby the rate of diffusion was small, and it was hardly useful for e.g. an adsorbent for a gas phase.
Further, in a case where a carbonized product of coal, a coconut shell, a resin or the like is to be activated by steam or the like in a process for producing active carbon, if the starting material is in the form of large particles, the activating gas tends to hardly diffuse into the interior of the particles, whereby it has been difficult to uniformly activate the material. Accordingly, a carbon material having a large particle size wherein mesopores are present in a substantial amount, has been desired. Further, with conventional active carbon, it has been technically rather difficult to mold its particles into a spherical or plate form, and active carbon which can be shaped or molded into an optional shape, has been desired.
Recently, a carbon aerogel having a large amount of uniform mesopores, has been developed. This carbon aerogel is agglomerates of ultrafine spherical carbon particles prepared by R. W. Pekala et al. (J. Non-Cryst. Solids, 145, 90, 1992, U.S. Pat. No. 4,873,218), and its structure is somewhat similar to a silica gel. Macromolecularly, it is a black gel substance having a gloss, and it is known to have a specific surface area of from 400 to 1,000 m.sup.2 /g. The structure of pores of the carbon aerogel has been analyzed mainly by a gas adsorption method. The carbon aerogel has a structural feature such that ultrafine carbon particles are linked to one another, and mesopores of from 20 to 500 .ANG. are uniformly formed by the linkage. It is believed that it further has micropores of not larger than 20 .ANG. in the ultrafine particles. However, the amount of micropores of the carbon aerogel is said to be small at a level of about 10% of the amount of micropores of usual active carbon obtainable by activating coal, a coconut shell or the like with steam or a reagent, whereby there has been a problem that the amount of micropores is not sufficient for its use as an adsorbent for a gas phase.
Pekala et al. further disclose activation of such a carbon aerogel with CO.sub.2 in WO 95/06002.
On the other hand, it has recently been attempted to use active carbon for a capacitor. Such a capacitor may be used, for example, as mounted on an electric car wherein a cell is used to generate electricity usually by a chemical reaction and it is usually difficult to supply a large quantity of electric current immediately when such a large quantity of electric current is required, for example, at the time of acceleration or initiation in driving, so that the electricity wanted can be supplied from the capacitor which physically stores such electricity (which is capable of discharging rapidly) when such a large quantity of electric current is required. For such a purpose, active carbon is required to be capable of being charged and discharged rapidly and have a large capacitance, when used as a capacitor.
With conventional active carbon, the properties when it is used as a capacitor, are not fully satisfactory, and further improvements have been desired.